1. Field of the Invention
This invention relates to the formation of toner images on a receiving sheet using an intermediate image member. It is particularly usable in color electrophotographic systems in which the intermediate image member is used to accumulate a series of single color images in registration to form a multicolor image and the multicolor image is transferred in a single step to the receiving sheet.
2. Description Relative to the Prior Art
Although other methods of transfer of toner images are known, by far the most common commercially used method of transferring a toner image from one surface to another is electrostatic. An electric field is applied to a toner image of a size and direction causing the toner particles to transfer. Traditionally, the most common method of creating an electric field for transfer is to spray corona on a reverse side of a sheet or web engaged in the transfer.
However, for a number of reasons the use of one or more backing rollers or similar members to create the electric field has a number of advantages. For example, in color systems, a receiving sheet can be attached to a drum and the drum rotated to bring the sheet through transfer relationship with a primary image member. An electric field between the drum and the image member superposes a series of single color images on the sheet creating a multicolor image. See, for example, U.S. Pat. No. 4,712,906, Bothner et al, issued Dec. 15, 1987 which is representative of a large number of references in commercial apparatus using this approach.
U.S. Pat. No. 3,781,105 granted to Meagher Dec. 25, 1973 suggests a backing roller for transferring single color images to a receiving sheet. In this instance the reference suggests that the backing roller have an outside layer or layers of a low intermediate conductivity and that a constant current source be used for establishing an electric field. The intermediate conductivity is established by using material having a resistivity of 10.sup.9 to 10.sup.11 ohm-cm. This material is conductive enough to permit the establishment of an electric field but provides a relatively high impedance which causes the field to be less variable in response to variations in the receiving sheet. With such more resistant materials, receiving sheets can vary between paper and transparency stock and also as to thickness and ambient relative humidity without an unacceptable variation in the field that would cause insufficient transfer in some instances or electrical breakdown in others.
Backing rollers having a resistivity in the neighborhood of 10.sup.10 ohm-cm are commonly made by doping a high resistance polyurethane material with tiny conductive particles such as carbon, iron or other antistatic materials sufficiently to provide the conductivity needed.
Although such backing rollers having a high resistivity are considered preferred in such systems, they do generate problems. If the field is provided between two members that roll in contact with each other, the field is constantly being established through that rolling contact. The substantial resistance of the backing roller increases the time constant in establishing the field thereby either increasing the necessary size of the nip for transfer or reducing the speed of the system.
A number of references show the use of intermediates in both single color image formation and multicolor image formation. For example, FIG. 8 of the above mentioned U.S. Pat. No. 4,712,906 shows a series of single color images being formed on a primary image member. The single color images are transferred in registration to an intermediate roller to create a multicolor image on the surface of the roller. A multicolor image is then transferred in a single step to a receiving sheet at a position remote from the primary image member. This system is particularly advantageous in forming multicolor toner images, because the receiving sheet does not have to be attached to a roller for recirculation but can be fed along a substantially straight path. It can also be used with single color toner image formation for a number of other reasons including facilitating duplex and preventing contact between a primary image member and a receiving sheet which may contaminate the image member with paper fibers and the like.
U.S. Pat. No. 4,931,839 granted to Tompkins et al on Jun. 5, 1990 shows use of an intermediate web of relatively high intermediate conductivity which superposes single color toner images by transfer from a primary image member. The images are transferred to a receiving sheet which is backed by a conductive roller. Substantial impedance does not appear to be provided at this transfer to allow for variations in receiving sheet impedance.